The traditional way to make an HTTP connection is to find the IP
address of the host using DNS, and connect to port 80 of that host address.
In theory, there can be many IP addresses (so called 'A records')
associated with a hostname in DNS. The W3C
reference library demonstrates a
mechanism to use the additional A records for failover. But in
practice, popular clients do not consult these additional A records.
The result is a single point of failure in almost all HTTP
connections.

Some services make information available via the web, but not
addressable. For example, results of database queries using POST
(rather than GET) are not addressable. A items in a catalog put on the
web this way can't be linked to, and cannot participate in third-party
search services. This unfortunate choice by some information providers
reduces automation and scalability in the web.

It is also unfortunate that, for example, headings in HTML documents
are not addressable unless they are marked up as anchors explicitly.
See XML above.

On DNS

Almost all of the URL schemes used today rely on the Domain Name
System. DNS is a hierarchical, administratively controlled namespace.
It is essential that this namespace be held in the public trust and
administered accordingly.